Multilayer coated hard alloy cutting tool

ABSTRACT

The present invention concerns a tungsten carbide base cutting tools formed on sintered hard alloy substrate material. Multiple hard coatings are deposited on the Co-enriched surface layer of the substrate material, and Co-enriched surface layer occurs within a surface layer region that is 50 μm from an interface of said substrate material, wherein the Co content in said surface layer that is 5-10 μm from an interface of said substrate material is within a range from 15-25% by weight; wherein the Co content in said surface layer is more than the Co content in the core, and wherein the content of the carbides of Ti, Ta and Nb is lower than that in the core. The multilayer coating consists of a primary coating of TiCN, a secondary coating of Al 2  O 3  and the surface coating consisting of at least one of TiCN and TiN. The interface between the substrate material and the primary coating is provided with a first intermediate coating consisting of TiN. The interface between the primary coating and the secondary coating is provided with a second intermediate coating consisting of at least one layer of TiC, TiCO or TiCNO.

This is a continuation of application Ser. No. 07/982,572, filed on Nov.27, 1992, now U.S. Pat. No. 5,374,471.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to hard alloy cutting tools havingmultilayer surface coatings for providing good adhesion, wear andchipping resistance.

2. Technical Background

The application of coated hard alloys for insert cutting tools (referredto as inserts hereinbelow) has been gaining popularity in recent years.For disposable inserts, the percentage of the coated tools has reachedabout 40% in Japan, and more than 60% in western countries.

One reason for such popularity for the coated inserts is the improvementin the toughness of the substrate materials.

It is known generally that when the surface of hard alloys is protectedwith a hard coating, although the wear resistance is improved, theresistance to chipping is degraded. To rectify this problem, it isessential to improve the toughness of the substrate material.

However, improving the toughness often means sacrificing the hardnesswhich provides a basis of wear resistance but is in a converserelationship to toughness.

For this reason, the past solutions for improving the toughness ofcoated hard alloys involved mainly the surface layer portion of thesubstrate material, not the substrate material itself. The concept isthat if the interior (core) of the hard alloys is hard, and the surfacelayers of the substrate material is tough, both wear resistance andchipping resistance can be improved simultaneously.

In fact, many of the coated hard alloy inserts in the markets forcutting steels and ductile cast irons, are made so that the surfacelayer is high in Co and has high toughness, and the core is relativelylow in Co and has high hardness.

Such materials were first disclosed in a Japanese Patent Application,First Publication, No. Sho 52(1977)-110,209, which disclosed a coatedhard alloy of improved toughness as a result of having a surface layerthickness of 10-200 μm, whose hardness is lowered by 2-20% compared withthat of the core of the substrate material.

In this patent application, the first embodiment shows a substratematerial of a composition, WC-10% TiC-10% Co (by weight in all thesubsequent cases, unless otherwise stated). coated with a slurry ofWC-10% Co, dried and sintered at 1430° C. for one hour to prepare asurface layer thickness of 130 μm, Vicker's hardness of 1320 in thesurface layer, and 1460 in the core. There are no TiC particles, whichare brittle, in the surface layer and the volume percent of the Co phasein the surface layer is higher than that in the core. A chemical vapordeposited (CVD) TiC coating of a 6 μm thickness is provided on theCo-enriched surface layer, thereby producing a coated high toughnesshard alloy.

In the second embodiment, a TiC coated hard alloy is presented in whicha mixture consisting of WC-6% Co and WC-10% Co is press, compacted andsintered to produce a substrate material having a surface layerthickness of 80 μm, and Vicker's hardness of 1320, and a core Vicker'shardness of 1450.

The embodiments of the Japanese Patent Application, First PublicationNo. Sho 53(1978)-131,909, involves a sintered hard alloy having a Cosurface coating, to produce a sintered hard alloy with a Vicker'shardness of 1050 at the surface. Another embodiment of the above-notedpatent application, involves the steps of preparing the mixture, a firstsintering, coating the surface with graphite and a second sintering, toproduce a toughness substrate material having a Vicker's hardness of1160 at the surface.

One of the programs of producing a coated hard alloy according to theabove noted Applications (No. 110,209 and No. 131,909) is the high cost.An excellent example of prior art proposed for solving this problem,there is U.S. Pat. No. 4,277,283.

The U.S. Pat. No. 4,277,283 (JPA, First Publlcation,Sho 54(1979)-No.87,719, discloses in the claims, an example of a coated substratematerial having high toughness in surface layers with a thickness of5-200 μm, in which the proportion of the B-1 type hard phases, TiC, TaCand TiN containing W, in the surface layer, is lower compared with thatin the core.

The first embodiment of the above-noted patent, discloses a sinteredhard metal, produced from a powdered mixture consisting of WC-4% (Ti₀.75W₀.25)(C₀.68 N₀.32)-5% (Ta₀.75 Nb₀.25)C-5.5% Co, and by heating themixture in a 1×10⁻³ vacuum at 1450° C. to eliminate the B-1 type hardphase completely to a depth of 10 μm, so that the surface layer isvirtually all WC-Co. The surface of the substrate material is coatedwith a 6 μm thick CVD TiC coating to produce a coated hard alloy cuttingtool. The toughness of this tool is high because the surface layerbecomes enriched with Co as the B-1 type hard phase is eliminated.

However, when the hard alloy cutting tool according to U.S. Pat. No.4,277,283 was used as a toll for high speed heavy cutting, it was stillfound to be insufficient due to failures such as breakage, because theCo enriched surface layer had a Co content 1.5-2.5 times greater thanthe average Co content of the core.

Another U.S. Pat. No. 4,610,931 discloses a coated hard alloy similarthe coated hard alloy disclosed in U.S. Pat. No. 4,277,283.

This patent further discloses the following: hard alloys containing nofree carbon particles in which a rake surface is removed by grinding andre-treated with heat to covert the nitrides and carbonitrides in thesurface layer to carbides; a Co-enriched surface hard alloy; and theabove-treated and coated hard alloy.

The first embodiment of this patent shows a material WC-10.3% TaC-5.85%TiC-0.2% NbC-8.5% Co-1.5% TiN, which is heated at 1496° C. for 30minutes; sintered in a vacuum; made into a cutting insert after whichthe upper and lower surfaces (rake surfaces) are ground; heated again at1427° C. for 60 minutes in a vacuum at 100 μm Hg, and after cooling at agiven rate to 1204° C., the flank surface is ground. The surface iscoated with TiC and TiN coatings using the usual CVD coating method toproduce coated hard alloys having no free carbon particles, and having aCo-enriched layer and no B-1 type hard phases to a depth of 22.9 μm, andcoated with a multilayer consisting of 5 μm thick TiC, 3 μm thick TiCNand 1 μm thick TiN layers.

Another U.S. Pat No. 4,830,930 (corresponding JPA, First Publication Sho63(1988)- No.169,356) to increase the Co content of the surface layer,discloses in the claims, a hard alloy substrate material, in which thesurface layer having a thickness of 100 to 500 μm, contains a gradientof a binder phase (Co-containing phase) such that the binder phaseconcentration is highest at the surface and levels off to a depth of 5μm towards the core.

The first embodiment of the above-noted patent discloses a method ofproducing a substrate material by the following steps: preparingcompacts of a powder mixture of WC-5% TiC-7% Co; sintering the compactsat 1380° C. for one hour, carburizing at 1330° C. for 10 minutes in anatmosphere of a 20 torr 80% H₂ -20% CH₄ mixture; decarburizing at 1310°C. for 2 minutes in an atmosphere of 10 torr 90% H₂ -10% CO₂ mixture;and cooling in a vacuum; thereby obtaining a microstructure having a Cocontent which is highest at the surface and gradually decreases towardsthe core Co content. The substrate material prepared in this manner iscoated with a CVD TiC coating of a 5 μm thickness.

However, the Co-enriched surface layer of the hard alloy cutting toolaccording to U.S. Pat. No. 4,830,930 has a Co content 2.2 times greaterthan the average Co content of the core.

The third embodiment of the above-noted patent discloses, a hard alloycomprising a surface and an inner portion, characterized in that theconcentration of the Co-enriched layer is highest (relative Co contentis 380%) at the outermost surface of the body and approaches theconcentration of the inner portion.

The preferred embodiments of the above-noted application discloses, acoated hard alloy material with a Co-enriched surface layer, having 2 μmof TiC, 2 μm of TiCN and 2 μm of TiN produced according to the chemicalvapor deposition method.

The hard alloy cutting tool according to U.S. Pat. No. 4,830,930discloses a coated hard alloy material having a Co-enriched surface onwhich the first coating of TiC is deposited. This hard alloy cuttingtool is deficient in that the wear resistance is inadequate due tointer-diffusion of WC and Co from the surface layer into the first TiCcoating. The reason recited for using the first layer of TiC is thatwhen TiC is applied directly to the Co-enriched surface layer, alloyingoccurs in the enriched layer.

In the following section, research studies for increasing the Co contentof the surface layer will be reviewed. A representative example is U.S.Pat. No. 4,911,989 (corresponding JPA, First Publication No. Hei63(1988)- No.197,569). U.S. Pat. No. 4,911,989 discloses asurface-coated and cemented carbide substrate in which the hardness ofthe cemented carbide substrate, in the range of 2-5 μm from theinterface between the coating layer and substrate, is 700-1300 kg/mm² byVicker's hardness (claim 1). The above noted hardness is less than onthe inside of the cemented carbide substrate. The above noted U.S. Pat.No. 4,911,989 further discloses that: the cemented carbide substrate,wherein the quantity of the Co in the cemented carbide substrate, in therange of 2-20 μm to 50-100 μm from the interface, is 1.5 to 7 times byweight greater than the average quantity of the Co (claim 5).

In this patent application, the fourth embodiment shows a substratematerial made of a powdered mixture, WC-2% TiCN-3% TaC-5.6% Co. which isheated in a vacuum to 1400° C., held for 30 minutes and is sintered in aN₂ atmosphere at 2 torr, cooled, to 1320° C. at a cooling rate of 10°C./min, and then cooled to 1200° C. in a vacuum (1×10⁻³ torr) at acooling rate of 1° C./min, to produce a cemented carbide substrateinterface where the quantity of the Co is 3 to 7 times, by weightgreater than the average quantity of the Co.

This above-noted cemented carbide substrate has no coating. However, thefirst embodiment concerns the surface coating of a cemented carbidesubstrate with the usual CVD TiC coating to this is of 5 μm thick andAl₂ O₃ coating that is 5 μm thick. The fifth embodiment concerns thesurface coating of a cemented carbide substrate with the usual CVDcoating TiC (3 μm)/TiN (2 μm)/TiC (1 μm)/Al₂ O₃ (1 μm). The seventhembodiments concern the surface coating of a cemented carbide substratewith the usual CVD coating TiC (5 μm)/Al₂ O₃ (1 μm).

However, the technology of the surface-coated hard alloy cutting tooldisclosed in U.S. Pat. No. 4,911,989 and U.S. Pat. No. 4,830,930 isdeficient because the wear resistance is inadequate, due to theinter-diffusion of WC and Co from the surface layer into the first TiCcoating.

The foregoing extensive review of the prior art technologies is given toshow that the studies are mostly concerned not with improving thecoatings, but with improving the toughness of the surface layer, whichprovides improved chipping resistance however still left a problem oflow wear resistance.

In the following section, research studies for improving the propertiesof the coatings will be reviewed. Representative examples are U.S. Pat.No. 4,497,874 and U.S. Pat. No. 4,812,370 (corresponding JPA, FirstPublication, No. Sho 63(1988)- No.89666).

U.S. Pat. No. 4,497,874 discloses a coated hard alloy material having aCo-enriched surface on which a first coating of TiN is deposited. Thereason recited for using the first layer of TiN instead of the usualcoating of TiC is if TiC coating is applied directly to the Co-enrichedsurface layer, alloying occurs in the enriched layer. Therefore, thefirst TiN coating is used to prevent such alloying, and to form a thicklayer of TiC without resorting to forming a gradation layer.

In the first embodiment of the above-noted patent, a method is disclosedfor preparing a substrate material of WC-6% TaC-6% Co-5% (W₀.5 Ti₀.5)C,according to the following steps: preparing pressed compacts anddewaxing at 1260° C.; heating the dewaxed compacts in a N₂ atmosphereand flowing rate of 3 l/min for 45 minutes; removing the nitrogen andraising the temperature to 1445° C., and sintering the compacts for 100minutes; to produce a substrate material having a Co-enriched 30 μmthick surface layer in which there is no B-1 type hard phase. The hardalloys are produced by coating the substrate material with TiN/TiC/TiNor with Al₂ O₃.

U.S. Pat. No. 4,812,370 (JPA No. Sho63(1988)- No.89666) discloses in theclaims, a coated hard alloy having a Co-enriched surface layer on whicha WC and Co-diffused TiC first coating is deposited, a TiCN/TiN secondcoating to prevent the diffusion of WC and Co, a third coating of pureTiC, and a fourth coating, such as TiCO, TiCNO and Al₂ O₃.

The preferred embodiments of the above-noted application disclose, acoated hard alloy material of WC-12.4% (Ti₀.46 Ta₀.22 W₀.32)(C₀.80N₀.20)-8.0% Co, having a Co-enriched surface layer of an 18 μmthickness, and having a 3 μm thick TiC coating with diffused WC and Co,2 μm of TiCN, 2 μm TiC and 0.3 μm Al₂ O₃ coating.

The foregoing technologies are aimed at solving the problems of thechipping of hard alloys when a CVD coating is applied directly to theCo-enriched surface layer of a substrate material, causing the formationof undesirable microstructures such as pores and a brittle eta phase inthe surface layer, due to the diffusion of WC and Co from the substrate.The TiC coatings with diffused WC and Co also suffer from poor wearresistance.

Even though the step of decarburizing disclosed in claims 11, 12 and 15of U.S. Pat. No. 4,497,874 may be applied before the first coating ofTiN is applied to the substrate material, the hard alloy producedaccording to this patent still presents problems such as the pooradhesion of the first coating TiN to the substrate material. And thewear resistance of said hard alloy produced according to this patent isnot significantly improved because the primary coating is TiC.

The technology disclosed in U.S. Pat. No. 4,812,370 (corresponding JPANo. Sho 63(1988)- No.89666) is also deficient in that the wearresistance is inadequate because of inter-diffusion of WC and Co fromthe surface layer into the first TiC coating.

To rectify such problems in the existing coated hard alloys as outlinedabove, the present invention presents a new technology for preparing acoated hard alloy cutting tool of high toughness and high resistance towear and chipping.

SUMMARY OF THE INVENTION

The objective of the present invention is to present a coated hard alloycutting tool of high toughness and high resistance to wear and chipping,in which the surface layer of the substrate material is free of poresand a brittle phase, and is adhered tightly to the coatings appliedthereon.

The present invention concerns a coated hard alloy cutting toolcomprising a plurality of hard coatings formed on the surfaces of aprimarily WC substrate material containing Co, and consistingessentially of a core and multiple surface layers.

A Co-enriched surface layer occurs within a surface layer region of 50μm from an interface of said substrate material, wherein the Co contentin a surface layer of 5-10 μm thickness from an interface of saidsubstrate material is within a range from 15-25% by weight, wherein theCo content in said surface layer is more than the Co content in saidcore, wherein the content of the carbides of Ti, Ta, and Nb is lowerthan that in said core, and wherein the plurality of surface coatingsconsist of a primary coating of TiCN deposited on the surface layer, asecondary coating of Al₂ O₃ deposited on the primary coating, and asurface coating consisting of at least one coating of TiCN and TiNdeposited on the secondary coating of Al₂ O₃.

A surface region within a distance of 100 μm to 400 μm from saidinterface between coatings and substrate material is substantially freeof said free carbon particles, while free carbon particles are presentin a region of said core located beyond 100-400 μm from said interfaceof said substrate material.

The Co content in a region of said core located substantially beyond 100μm depth from said interface of said substrate material is within therange from 4-8% by weight. The proportion of a carbide of Ti, Ta and Nbat the surface layer of 5-10 μm thickness from said interface of saidsubstrate material is lower than that in the core portion at a depth of100 μm of said substrate material.

The coatings of the present invention are deposited at relatively lowtemperatures, and a relatively high concentration of Co in the surfacelayers. Therefore, compared with the existing coated cutting tools,residual tensile stresses in the as-deposited coating layers are heldrelatively low, between not more than 20 Kg/mm². The low residual stresslevel in the coatings is a reason for high chipping resistance of thecutting tools of the present invention.

The interface (which is also the internal surface of the substratematerial) between the substrate material and the primary coating of TiCNis provided with a first intermediate coating of TiN to lower theresidual stress of the primary coating of TiCN. The thickness of thefirst intermediate coating of TiN (between the substrate interface andthe primary coating TiCN) is also preferably less than 1 μm.

Between the primary coating and the secondary coating, a secondintermediate coating, consisting of at least one layer of a TiC layer,TiCO layer or TiCNO layer, is provided so as to improve the adhesion ofthe coatings. The thickness of the second intermediate coating of TiCOor TiCNO layer that are preferably less than 1 μm thickness.

However, the coated hard alloy cutting tool that has coated directly onthe surface of the substrate material will still poor adhesion betweenthe coating layer and the substrate material. Accordingly, a barrelfinishing, a shot blasting and/or acid dipping is applied to the surfaceof the substrate material after sintering, to solve the above notedproblem.

Other variations of the basic invention includes the followingvariations.

The chipping resistance is improved further in the present invention bytreating the as-deposited coatings so as to adjust the magnitude andtype of residual stresses in the coatings. In some cases, the tensileresidual stresses in the coating can be converted into compressiveresidual stresses. This is accomplished in the following way.

Shot peening is employed in the present invention to effectively controlthe magnitude and type of residual stresses in the shot peened coatingsand underlying coating. By this processing, the tensile residual stresslevel is lowered to not more than 10 Kg/mm², and by varying the peeningconditions, it is possible to convert tensile stresses into compressivestresses of not more than 20 Kg/mm².

By impacting the surfaces of the coated alloy with steel balls therebylowering the tensile residual stresses therein, chipping resistance ofthe coated alloy is increased. However, wear resistance is lowered insome cases. Therefore, the shot peening process is applied locally toparts of the cutting tool, for example to the rake surfaces, so that theresidual tensile stresses in the primary coating thereon are lower thanthose tensile residual stresses in the primary coating on the flanksurfaces of the cutting tool.

Further shot peening treatment is applied so that the residual stressesin the primary coating of the rake surfaces of the cutting tool arecompressive, and that the residual stresses in the primary coating ofthe flank surfaces are tensile.

It is effective to treat only the rake surfaces, such a procedure isalso more economical for production purposes also. By so doing, chippingresistance of the coated alloy increases, and lowering in wearresistance becomes rare.

Also in the above-noted structures of cutting tools also, the residualtensile stresses in the primary coating can be made to be not more than20 Kg/mm². This value can be further controlled with the application ofshot peening to not more than 10 Kg/mm². With further peening, it iseven possible to convert the tensile residual stresses in the primarycoating to compressive residual stresses, and to control the value ofthe compressive residual stresses so that it is not more than 20 Kg/mm².

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an example of application of the presentinvention to making of an insert.

FIG. 2 is a cross sectional view of the coating configuration of a firstembodiment of the insert shown in FIG. 1.

FIG. 8 is a cross sectional view of the coating configuration of asecond embodiment of the insert shown in FIG. 1.

FIG. 4 is a cross sectional view of the coating configuration of a thirdembodiment of the insert shown in FIG. 1.

FIG. 5 is a cross sectional view of the coating configuration of afourth embodiment of the insert shown in FIG. 1.

FIG. 6 is a cross sectional view of the coating configuration of a fifthembodiment of the insert shown in FIG. 1.

FIG. 7 is a cross sectional view of the coating configuration of a sixthembodiment of the insert shown in FIG. 1.

FIG. 8 shows a relationship between the Co concentration and thedistance from the interface of the substrate material in some samples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Basic Configurations

FIG. 1 is an example of applying the technique of preparing the coatedhard alloy material of the invention to an insert. A square-shapedinsert body 1 has a rake surface 2 on each of the top and bottomsurfaces, and the flank surfaces 3 are formed on the side surfacesthereof, forming cutting edges 4 at the intersections of the top andbottom surface with the side surfaces. The insert body 1 comprises asubstrate material and various coatings to be described later.

In this embodiment, a square shape is illustrated, but the inventedstructural configuration is equally applicable to other shapes such astriangles, parallelpipeds, rhomboids and circles.

FIG. 2 is a first embodiment of the coating layer configuration of theinvention. The coating layer 10 of this embodiment is formed on asubstrate material 12, and consists of a primary coating 13, a secondarycoating 14 and a surface coating 15.

FIG. 3 is a second embodiment of the coating layer configuration of theinvention. The coating layer 20 of this embodiment is formed on theinterface of the substrate material 12, and consists of a firstintermediate coating 16, the primary coating 13, the secondary coating14 and the surface coating 15.

FIG. 4 is a third embodiment of the coating layer configuration of theinvention. The coating layer 30 of this embodiment is formed on theinterface of the substrate material 12, and consists of a firstintermediate coating 16, the primary coating 13, a second intermediatecoating 17, the secondary coating 14 and the surface coating 15.

FIG. 5 is a forth embodiment of the coating layer configuration of theinvention. The coating layer 40 of the embodiment is formed on theinterface of the substrate material 12, and consists of the primarycoating 13, the second intermediate coating 17, the secondary coating 14and the surface coating 15.

FIG. 6 is a fifth embodiment of the coating layer configuration of theinvention. The coating layer 50 of the embodiment is formed on theinterface of the substrate material 12, and consist of the primarycoating 13, the second intermediate coating 17, the secondary coating 14and the surface coating 15. The second intermediate coating 17 consistsof a primary intermediate coating 18 and a secondary intermediatecoating 19.

FIG. 7 is a sixth embodiment of the coating layer configuration of theinvention. The coating layer 60 of the embodiment is formed on theinterface of the substrate material 12, and consist of the firstintermediate coating 16, the primary coating 13, the second intermediatecoating 17, the secondary coating 14 and the surface coating 15. Thesecond intermediate coating 17 consists of a primary intermediatecoating 18 and a secondary intermediate coating 19.

The substrate material 12 has WC as its primary constituent, with Coadded as a binder, but may contain other additives such as B-1 type hardphases comprising carbides, nitrides and/or carbonitrides of Ti, Ta andNb containing W, and unavoidable impurities. However, the essentialconditions are that the Co-enriched surface layer occurs within asurface layer region of 50 μm from an interface of said substratematerial, wherein the Co content in a surface layer that is 5-10 μmthickness from an interface of said substrate material is within a rangefrom 15-25% by weight; wherein the Co content in said surface layer ismore than the Co content in said core; and wherein the content of thecarbides of Ti, Ta and Nb is lower than that in said core. In addition,another essential condition is that the Co content in the region of saidcore located beyond about 100 μm depth from said interface of saidsubstrate material is within the range from 4-8% by weight.

A surface region within a distance of 100 μm to 400 μm. from saidinterface between coatings and substrate material is substantially freeof said free carbon particles, while free carbon particles are presentin a region of said core located beyond 100-400 μm from said interfaceof said substrate material.

The primary coating 13 is composed of a TiCN layer, the secondarycoating 14 is composed of a Al₂ O₃ layer, and the surface coating 15 iscomposed of either or both of a TiCN layer and a TiN layer. The firstintermediate coating 16 is composed of a TiN layer and the secondintermediate coating is composed of at least one of the layers of TiC,TiCO and TiCNO.

The procedure for preparing the substrate material 12 will be describedin the following. A powder mixture corresponding to the desiredcomposition of the substrate material 12 is prepared. This powdermixture is mixed with binders and additives, as necessary, and themixture is ball-milled and dried to obtain a powder material. The powdermaterial which can be used in preparing the raw material includes anyone or a plurality of the elements in Group 4a, Group 5a and Group 6a;or carbides, nitrides and carbonitrides of Group 4a, Group 5a and Group6a elements as well as other known elements or compounds generally usedin hard alloy materials, such as powdered materials of WC, (W,Ti)C,(Ta,Nb)C, Co and graphite.

Next, the powdered material is press compacted into green compacts,which are sintered in a reduced pressure furnace at around 1400° C. toproduce a substrate material which has a core that contains free carbonparticles but whose surface layer of 100-400 μm depth is substantiallyfree of free carbon particles.

The free carbon particles are precipitated as black particles in thebody of the substrate material during carburizing, but in this inventionthis precipitation is controlled to occur in the core at the depth of100-400 μm, which is referred to as the core zone. In other words, theprecipitation depth closest point to the surface is 100 μm, and thefarthest depth is 400 μm.

Such microstructural changes can be observed readily with an opticalmicroscope, because the carbides of the above mentioned elements areetched black in the metallographic specimen preparation.

The surfaces of the sintered compacts are processed by such means ashoning, and CVD coatings deposited at relatively low temperature thereonto produce coated hard alloy having the surface layer with high Cocontent(15-25%) inserts of the invention. In depositing such coatings,the residual stresses in the as-deposited coatings are tensile, whosevalue is less than 20 Kg/mm².

After the coatings are applied, the residual stresses in the coatingscan be adjusted by means of shot peening. By adjusting the peeningparameters, the residual stresses can be lowered from tensile residualstress of 20 Kg/mm² to less than 10 Kg/mm². The stress type can also bealtered from a tensile to a compressive type. In practice, in the caseof steel balls, the speed is in a range of 50-70 m/s, and the peeningtime of 60-90 seconds to obtain the range of stresses mentioned above.

First Preferred Embodiment and Processing Steps

WC powder of 3.5 μm average diameter, (W₀.58 Ti₀.42)C powder of 1.5 μmaverage diameter, (Ta₀.83 Nb₀.17)C powder of 1.4 μm average diameter, Copowder of 1.2 μm average diameter, were blended into a mixture having acomposition, WC-8% (W₀.58 Ti₀.42)C-5.5% (Ta₀.83 Nb₀.17)C-5.5% Co, all byweight, to which 0.08% graphite powder was added, and the entire mixturewas wet-milled for 72 hours in a ball-mill, and dried. Green pressedcompacts were made in accordance with ISO CNMG120408 using a press at 15Kg/mm². The green pressed compacts were sintered in a vacuum of 1×10⁻²torr at 1410° C. for one hour. Samples of hard alloy substrate material,which were basically free of free carbon particles were thus produced.

These samples were carburized in a gas mixture of H₂ (80%)-CH₄ (20%, allby volume) at a reduced pressure of 10 torr, at 1400° C., for one hour,and cooled to room temperature at a cooling rate of 2.5° C./min. Thisone group of samples of the hard alloy substrate material, is has adenuded zone of 150 μm depth which is basically free of free carbonparticles and has a core zone of over 150 μm depth which has free carbonparticles when viewed under optical microscope.

The cutting edges were prepared by honing the surface to a depth of 0.07mm on the rake surface and to a depth of 0.04 mm on the flank surfaces.

These hard alloy samples were given a shot-blasting treatment by airpressure at 5 kg/cm², with a #1000 Al₂ O₃ powder, followed by submersionof the samples in a 0.3% nitric acid solution for 10 minutes. Thecoatings were applied under the conditions shown in Table 1 to producecoated hard alloy cutting insert samples 1 to 12 (hereinbelow termedsamples) listed in Table 2.

The profiles of the concentration gradient of Co in the coated hardalloy insert samples are shown in FIG. 8. These results were obtained byenergy dispersive X-ray spectroscopy in a 4×26 μm area under a scanningelectron microscope at a magnification of 5,000. The measurements wererepeated five times at a designated depth to obtain an average value.

In these samples, the residual stress values in the primary TiCN coatingdetermined by a X-ray technique are as shown in Table 2.

A part of the coated samples was subjected to shot peening using 0.3 mmdiameter steel shot at a speed of 50 m/s for 60 seconds, to produce thecoated samples of the present invention shown in Table 2. The residualstresses in the TiCN coating were also measured, as reported In Table 2.

By this kind of process, a substrate material was obtained in which theTi content was 0.9%, the Ta content was 1.6% and the Nb content was 0.2%at a depth 10 μm, and in which the Ti content was 2.0%, the Ta contentwas 4.6% and the Nb content was 0.5% at a depth of 100 μm. Accordingly,the B-1 type hard phase content at the surface of said substratematerial was less than the hard phase content of the internal portion ofthe substrate material.

For comparative evaluation purposes, samples A shown in Table 2 wereproduced, according to the process disclosed in U.S. Pat. No. 4,277,283(JPA, First Publication No. Sho 54(1979)- No.87719). These samples wereproduced for comparative evaluation by blending starting materials frompowdered particles of: WC-6.3% (Ti₀.75 W₀.25)(C₀.68 N₀.32)-7.5% (Ta₀.75Nb₀.25)C-10.5% Co, with 0.26% graphite, and by pressing the powder toproduce green pressed compacts. They were sintered at 1380° C. in avacuum of 1×10⁻³ torr to produce samples of a substrate material havingessentially free carbon particles. The samples were treated by honingand a 6 μm thick coating of TiC was deposited thereon using the sameprocedure as the first embodiment to produce sample A for comparativeevaluation.

The profiles of Co distribution in the surface layer of the substratematerial were as shown in FIG. 8, indicating that the thickness of theB-1 phase denuded zone was 19 μm and that the substrate material hadessentially no Ti, Ta and Nb at the depth of 10 μm from the surface ofsaid substrate material.

Additional samples were produced for comparative evaluation according tothe first embodiment disclosed in JPA, First Publication No. Sho63(1988)- 169,356.

The substrate material of this disclosed embodiment was WC-5% TiC-7% Co,and after blending the materials and pressing to produce green pressedcompacts, they were sintered at 1380° C. for 1 hour, in a vacuum. Theywere then carburized in a gas mixture of H₂ (80%)-CH₄ (20%, by volume)at a reduced pressure of 20 torr, and subsequently decarburized at 1310°C., for 2 minutes, in a gas mixture of H₂ (90%)-CO₂ (10%, all byvolume), and cooled to room temperature in a vacuum.

The substrate material thus produced was treated by honing and a coatingof TiC was deposited by the same procedure as in the first embodiment toproduce sample B with a 5 μm thick coating of TiC. The profile of the Codistribution was as shown in FIG. 8, indicating that the B-1 type hardphase was present in the surface layer and in the core zone. The Ticontent of the substrate material was 4.5% by weight at a depth of 10μm, and 5.1% at a depth of 100 μm depth.

Also for comparative purposes, coated hard alloy insert sample C wasprepared in the same way as disclosed in Example 1 of U.S. Pat. No.4,497,874.

This sample for comparative evaluation was produced by mixing thestarting material from powdered particles of: WC-5% (W₀.5 Ti₀.5)C-6%TAG-6% Co, with 0.08% graphite. This mixture was then press compacted,dewaxed and sintered at 1260° C. with flowing nitrogen, at a rate of 3l/min, at a reduced pressure of 600 torr. After 45 minutes of heating,the nitrogen was removed and sintering was performed at 1445° C. for 100minutes in a reduced-pressure argon atmosphere of 2 torr. The surfacesof the sample were honed as before, and a multilayer coating consistingof TiN (1.5 μm)-TiC (8 μm)-Al₂ O₃ (2 μm).

The thickness of the B-1 phase denuded zone was 28 μm, and the presenceof free-carbon particles was noted. This sample had essentially no Ti,Ta at a depth of 10 μm.

In addition sample D was produced for comparative evaluation inaccordance with the first embodiment in the U.S. Pat. No. 4,610,931.

The substrate material of this disclosed embodiment was WC-10.3%TAC-5.85% TiC-0.2% NbC-1.5% TiN-8.5% Co, to which 0.1% graphite powderwas added, and after blending the materials and pressing to producegreen pressed compacts, they were sintered at 1496° C., for 30 minutes,in a vacuum. There after, only the rake surfaces (top and bottomsurfaces) were ground. The sample was then vacuum-heated at 1427° C.,for 1 hour, in a vacuum of 100 μmHg, cooled, at a rate of 56° C./min, to1204° C.; and cooled to room temperature in a vacuum. The flank surfaceswere then ground, and a CVD coating TiC (5 μm)/TiCN (4 μm)/TiN (1 μm)was deposited thereon. The profile of the Co distribution is as shown inFIG. 8, indicating that the thickness of the denuded B-1 phase zone was20 μm, and that this sample had essentially no Ti, Ta at a depth of 10μm.

All of these samples produced for comparative evaluation were subjectedto X-ray residual stress determinations.

                  TABLE 1                                                         ______________________________________                                                   Gas Composition Reaction T                                         Coating    (volume %)      (°C.)                                       ______________________________________                                        TiCN       TiCl.sub.4 1.5       860                                           (for       CH.sub.3 CN                                                                              0.5                                                     primary    N.sub.2    25                                                      coating)   H.sub.2    remainder                                               Al.sub.2 O.sub.3                                                                         AlCl.sub.3 5.0      1020                                                      CO.sub.2   8.0                                                                H.sub.2    remainder                                               TiCN       TiCl.sub.4 2        1020                                           (for       CH.sub.4   5                                                       surface    N.sub.2    20                                                      coating)   H.sub.2    remainder                                               TiC        TiCl.sub.4 2        1020                                                      CH.sub.4   5                                                                  H.sub.2    remainder                                               TiN        TiCl.sub.4 2        1020                                                      N.sub.2    30                                                                 H2         remainder                                               TiCO       TiCl.sub.4 2        1020                                                      CO         6                                                                  H.sub.2    remainder                                               TiCNO      TiCl.sub.4 2        1020                                                      CO         3                                                                  N.sub.2    3                                                                  H.sub.2    remainder                                               ______________________________________                                    

Next, machining tests were carried out using the samples of the presentinvention as well as those of the comparative evaluation thus produced.Continuous machining tests:

Material machined: a cylinder of JIS SCM440 (HB 200)

Machining speed: 220 m/min

Feed rate: 0.35 mm/rev.

Depth of Cut: 2.0 mm

Machining duration: 30 minutes

Lubricant: water soluble

Interrupted machining tests:

Material machined: a square cylinder of JIS SNCM439 (HB 270)

Machining speed: 100 m/min

Feed rate: 0.375 mm/rev.

Bite: 3.0 mm

Lubricant: Done

In continuous machining, the wear of the rake surface was measured, andin interrupted machining, the resistance to chipping was evaluated bythe time until the occurrence of the first chipping event.

                  TABLE 2                                                         ______________________________________                                                                   Residual                                                                             Wear  Chipping                              Test                       Stress Width Time                                  No.   Coating    Peening   (Kg/mm.sup.2)                                                                        (mm)  (min)                                 ______________________________________                                        1     TiCN(9.5)- None      TiCN/15T                                                                             0.28  18.9                                        Al.sub.2 O.sub.3 (2)-                                                         TiN(1)                                                                  2     TiCN(6)-   None      TiCN/14T                                                                             0.27  18.1                                        TiC(3.5)-                                                                     Al.sub.2 O.sub.3 (2)-                                                         TiN(1)                                                                  3     TiCN(9.5)- None      TiCN/14T                                                                             0.25  18.9                                        TiCNO(0.3)                                                                    Al.sub.2 O.sub.3 (2)-                                                         TiCN(1)                                                                       TiN(1)                                                                  4     TiCN(6)-   None      TiCN/16T                                                                             0.23  18.1                                        TiC(3.5)-                                                                     TiCNO(0.3)-                                                                   Al.sub.2 O.sub.3 (2)-                                                         TiN(1)                                                                  5     TiN(0.5)-  None      TiCN/13T                                                                             0.24  18.7                                        TiCN(9)-                                                                      TiCNO(0.3)-                                                                   Al.sub.2 O.sub.3 (2)-                                                         TiCN(1)-                                                                      TiN(1)                                                                  6     TiCN(9.5)- All       TiCN/4T                                                                              0.30  21.8                                        Al2O3(2)-  Surfaces                                                           TiN(1)                                                                  7     TiCN(9.5)- All       TiCN/2T                                                                              0.27  21.7                                        TiCNO(0.3)-                                                                              Surfaces                                                           Al.sub.2 O.sub.3 (2)-                                                         TiN(1)                                                                  8     TiCN(9.5)- Rake      TICN/  0.24  21.7                                        TiCNO(0.3) Surface   rake/2T                                                  Al.sub.2 O.sub.3 (2)-                                                                              flank/14T                                                TiN(1)                                                                  9     TiCN(6)-   Rake      TiCN/  0.23  21.0                                        TiC(3.5)-  Surface   rake/3T                                                  TiCNO(0.3)           flank/16T                                                Al.sub.2 O.sub.3 (2)-                                                         TiN(1)                                                                  10    TiN(0.5)-  All       TiCN/4C                                                                              0.26  21.5                                        TiCN(9)-   Surface                                                            TiCNO(0.3)-                                                                   Al.sub.2 O.sub.3 (2)-                                                         TiCN(1)-                                                                      TiN(1)                                                                  11    TiN(0.5)-  Rake      TiCN/  0.23  21.6                                        TiCN(9)-   Surface   rake/3C                                                  TiCNO(0.3)-          flank/13T                                                Al.sub.2 O.sub.3 (2)-                                                         TiCN(1)                                                                       TiN(1)                                                                  12    TiN(0.5)-  Rake      TiCN/  0.23  21.1                                        TiCN(6)-   Surface   rake/2T                                                  TiC(3)-              flank/15T                                                TiCNO(0.3)-                                                                   Al.sub.2 O.sub.3 (2)-                                                         TiCN(1)                                                                       TiN(1)                                                                  A     TiC(6.0)-  None      TiC/35T                                                                              0.68  6.7                                                                     in                                                                            6 min                                       B     TiC(5)-    None      TiC/33T                                                                              0.63  7.2                                                                     in                                                                            8 min                                       C     TiN(1.5)-  None      TiC/31T                                                                              0.58  3.8                                         TiC(8)-                     in                                                Al.sub.2 O.sub.3 (2)        14 min                                      D     TiC(5)-    None      TiCN/30T                                                                             0.54  7.0                                         TiCN(4)-                    in                                                TiN(1)-                     10 min                                      ______________________________________                                         Notes: In Table 2, various abbreviations are as follows:                      TiCN(9.5) indicates a TiCN coating of 9.5 μm thickness.                    TiCN/15T indicates a tensile residual stress value of 15 Kg/mm.sup.2          measured on a TiCN coating.                                                   TiCN/rake/3C flank/13T indicates residual stress values of that are           compressive residual stress of 3 Kg/mm.sup.2 measured on a rake surface,      and a tensile residual stress of 13 kg/mm.sup.2 measured on flank surface     of TiCN coating.                                                         

The results shown in Table 2 demonstrate clearly that the coated hardalloy insert according to the present invention are far superior tothose made by the comparative methods. The performance parameters, wearresistance and chipping tendencies are greatly improved over theconventionally prepared cutting tools.

The coated cutting tool of the present invention is characterized by aCo concentration gradient in the Co-enriched surface layer such that theCo-enriched surface layer occurs within a surface layer region of 50 μmfrom an interface of said substrate material, wherein the Co content ina surface layer of 5-10 μm thickness from an interface of said substratematerial is within a range from 15-25% by weight; wherein the Co contentin said surface layer is more than the Co content in said core; whereinthe content of the carbides of Ti, Ta and Nb is lower than that in saidcore said surface region within a distance of 100 μm to 400 μm from saidinterface between said coatings and said substrate material issubstantially free of said free carbon particles, while free carbonparticles are present in a region of said core located beyond 100-400 μmfrom said interface of said substrate material; and wherein theplurality of surface coatings consist of a primary coating of TiCNdeposited on the surface layer, a secondary coating of Al₂ O₃ depositedon the primary coating, and a surface coating consisting of at least onecoating of TiCN and TiN deposited on the secondary coating of Al₂ O₃.

The Co content in a region of said core located beyond approximately 100μm in depth from said interface of said substrate material is within therange of 4-8% by weight.

The primary coating on the invented cutting tool is TiCN, and is made byreacting titanium tetrachloride with acetonitrile at the relatively lowtemperatures of 840°-900° C., compared to 1000° to 1050° C. of theconventional technique.

Therefore, there is less diffusion of the elements constituting thesubstrate material, such as WC and Co, into the coating, and there isless tendency to form detrimental microstructural phases, such as poresand the brittle phases (eta phase), thereby improving the bonding of theprimary coating TiCN to the substrate material.

The technique of depositing a coating on a substrate material with theuse of TiCl₄ and acetonitrile is disclosed as an example in JPA, FirstPublication No. Sho 50 (1975)- No.117809, but the substrate material hasa composition, WC-22% (TiC+TaC)-9.5% Co, but has neither a Co-enrichedsurface nor a B-1 phase denuded zone, and is a typical conventionalmaterial which did not come into general use.

The present coatings, composed of primarily TiCN, are far superior tosuch materials because they are produced at relatively low depositiontemperatures, and are deposited on a substrate material having aCo-enriched surface layer and a core, wherein said Co-enriched surfacelayer occurs within a surface layer region of 50 μm from an interface ofsaid substrate material, wherein the Co content in a surface layer of5-10 μm from an interface of said substrate material is within a rangefrom 15-25% by weight, wherein the Co content in said surface layer ismore than the content in said core, and wherein the content of thecarbides of Ti, Ta and Nb is lower than that in the core. The surfaceregion within a distance of 100 μm to 400 μm from said interface issubstantially free of said free carbon particles, while free carbonparticles are present in a region of said core located beyond 100-400 μmfrom said interface of said substrate material.

It is generally known that in forming deposits by thin film formingtechniques, such as CVD, residual tensile stresses are generated in thecoating (TiC) because of the differences in the thermal coefficient ofexpansion between the coating layer and substrate material. The valuesof such residual stresses differ among the coatings, depending on thecoating thickness, and the composition of both coatings and substratematerials.

In the substrate material containing less than 10% Co. residual tensilestresses in a range of 30 to 60 Kg/mm² are reported to be present(Journal of the Japan Institute of Metal, v. 50, No. 3, pp320-327,1986).

In Table 2, it can be seen that the tensile residual stresses of theconventional materials all exceed 30 Kg/mm². However, in the presentinvention, coating layers are deposited at relatively low temperaturesand a relatively high concentration of Co in the surface layers.Therefore, compared with the existing coated cutting tools, residualtensile stresses in the as-deposited coating layers are held relativelylow, not more than 20 Kg/mm². The low residual stress level in thecoatings is a reason for high chipping resistance of the cutting toolsof the present invention. Therefore it was found that in the presentinvention, the residual stresses can be decreased, and by selecting thepeening conditions, tensile stresses in the deposited coatings can beconverted to compressive residual stresses.

What is claimed is:
 1. A coated hard alloy cutting tool, comprising:asubstrate comprising WC and Co, said substrate has a Co enriched surfacelayer of 50 μm from the external surface of said substrate and a core,the Co concentration of a region that is 5-10 μm from the externalsurface is from 15-25 wt %, the Co content of said surface layer is morethan that of said core, the content of carbides of Ti, Ta and Nb in thesurface layer is lower than the content of said core; and at least onecoating deposited on said surface of said substrate, said coating,sequentially consists of:(a) a primary layer comprising a TiCN layer;(b) a secondary layer comprising an Al₂ O₃ layer; and (c) a surfacelayer comprising at least one layer selected from the group consistingof a TiCN layer and a TiN layer; wherein said primary layer is producedso that tensile residual stresses therein are not more than 20 Kg/mm² ;and wherein said layers (a)-(c) are deposited by chemical vapordeposition.
 2. A coated hard alloy cutting tool as claimed in claim 1,wherein a surface region within a distance of 100 μm to 400 μm from saidexternal surface is substantially free of free carbon particles, whilefree carbon particles are present in a region of said core locatedbeyond 100-400 μm from said external surface of said substrate.
 3. Acoated hard alloy cutting tool as claimed in claim 1, wherein the Cocontent in a region of said core located beyond about a depth of 100 μmfrom said external surface of said substrate is within the range of from4-8% by weight.
 4. A coated hard alloy cutting tool as claimed in claim1, wherein said substrate is provided with rake surfaces and flanksurfaces, wherein the tensile residual stresses in said primary layer onsaid rake surfaces are not greater than tensile residual stresses insaid primary layer on said flank surfaces.
 5. A coated hard alloycutting tool, comprising:a substrate comprising WC and Co, saidsubstrate has a Co enriched surface layer of 50 μm from the externalsurface of said substrate and a core, the Co concentration of a regionthat is 5-10 μm from the external surface is from 15-25 wt %, the Cocontent of said surface layer is more than that of said core, thecontent of carbides of Ti, Ta and Nb in the surface layer is lower thanthe content of said core; and at least one coating deposited on saidsurface of said substrate, said coating, sequentially consists of:(a) aprimary layer comprising a TiCN layer; (b) a secondary layer comprisingan Al₂ O₃ layer; and (c) a surface layer comprising at least one layerselected from the group consisting of a TiCN layer and a TiN layer;wherein said substrate is formed with rake surfaces and flank surfaces;wherein said primary layer on said rake surfaces is treated to producecompressive residual stresses of not more than 20 Kg/mm² ; wherein saidprimary layer on said flank surfaces is produced so that tensileresidual stresses therein are not more than 20 Kg/mm² ; and wherein saidlayers (a)-(c) are deposited by chemical vapor deposition.
 6. A coatedhard alloy cutting tool, comprising:a substrate comprising WC and Co,said substrate has a Co enriched surface layer of 50 μm from theexternal surface of said substrate and a core, the Co concentration of aregion that is 5-10 μm from the external surface is from 15-25 wt %, theCo content of said surface layer is more than that of said core, thecontent of carbides of Ti, Ta and Nb in the surface layer is lower thanthe content of said core; and at least one coating deposited on saidsurface of said substrate, said coating, sequentially consists of:(d) afirst intermediate layer comprising a TiN layer; (a) a primary layercomprising a TiCN layer; (b) a secondary layer comprising an Al₂ O₃layer; and (c) a surface layer comprising at least one layer selectedfrom the group consisting of a TiCN layer and a TiN layer; wherein saidprimary layer is produced so that tensile residual stresses therein arenot more than 20 Kg/mm² ; and wherein said layers (a)-(d) are depositedby chemical vapor deposition.
 7. A coated hard alloy cutting tool asclaimed in claim 6, wherein a surface region within a distance of 100 μmto 400 μm from said external surface is substantially free of freecarbon particles, while free carbon particles are present in a region ofsaid core located beyond 100-400 μm from said external surface of saidsubstrate.
 8. A coated hard alloy cutting tool as claimed in claim 6,wherein the thickness of said first intermediate layer is not more than1 μm.
 9. A coated hard alloy cutting tool as claimed in claim 6, whereinthe Co content in a region of said core located beyond about 100 μmdepth from said external surface of said substrate is within the rangeof from 4-8% by weight.
 10. A coated hard alloy cutting tool as claimedin claim 6, wherein said substrate is provided with rake surfaces andflank surfaces, and wherein said primary layer on said rake surfaces isprovided with tensile residual stresses which are not greater thantensile residual stresses in said primary layer on said flank surfaces.11. A coated hard alloy cutting tool, comprising:a substrate comprisingWC and Co, said substrate has a Co enriched surface layer of 50 μm fromthe external surface of said substrate and a core, the Co concentrationof a region that is 5-10 μm from the external surface is from 15-25 wt%, the Co content of said surface layer is more than that of said core,the content of carbides of Ti, Ta and Nb in the surface layer is lowerthan the content of said core; and at least one coating deposited onsaid surface of said substrate, said coating, sequentially consistsof:(d) a first intermediate layer comprising a TiN layer; (a) a primarylayer comprising a TiCN layer; (b) a secondary layer comprising an Al₂O₃ layer; and (c) a surface layer comprising at least one layer selectedfrom the group consisting of a TiCN layer and a TiN layer; wherein saidsubstrate is formed with rake surfaces and flank surfaces; wherein saidprimary layer on said rake surfaces is treated to produce compressiveresidual stresses of not more than 20 Kg/mm² ; wherein said primarylayer on said flank surfaces is produced so that tensile residualstresses therein are not more than 20 Kg/mm² ; and wherein said layers(a)-(d) are deposited by chemical vapor deposition.
 12. A coated hardalloy cutting tool, comprising:a substrate comprising WC and Co, saidsubstrate has a Co enriched surface layer of 50 μm from the externalsurface of said substrate and a core, the Co concentration of a regionthat is 5-10 μm from the external surface is from 15-25 wt %, the Cocontent of said surface layer is more than that of said core, thecontent of carbides of Ti, Ta and Nb in the surface layer is lower thanthe content of said core; and at least one coating deposited on saidsurface of said substrate, said coating, sequentially consists of:(a) aprimary layer comprising a TiCN layer; (e) a second intermediate layercomprising at least one layer selected from the group consisting of aTiC layer, a TiCO layer and a TiCNO layer; (b) a secondary layercomprising an Al₂ O₃ layer; and (c) a surface layer comprising at leastone layer selected from the group consisting of a TiCN layer and a TiNlayer; wherein said primary layer is produced so that tensile residualstresses therein are not more than 20 Kg/mm² ; and wherein said layers(a)-(c) and (e) are deposited by chemical vapor deposition.
 13. A coatedhard alloy cutting tool as claimed in claim 12, wherein a surface regionwithin a distance of 100 μm to 400 μm from said external surface issubstantially free of free carbon particles, while free carbon particlesare present in a region of said core located beyond 100-400 μm from saidexternal surface of said substrate.
 14. A coated hard alloy cutting toolas claimed in claim 12, wherein the thickness of said secondintermediate layer of a TiCO layer or a TiCNO layer is not more than 1μm.
 15. A coated hard alloy cutting tool as claimed in claim 12, whereinthe Co content in a region of said core located beyond about 100 μmdepth from said external surface of said substrate is within the rangeof from 4-8% by weight.
 16. A coated hard alloy cutting tool as claimedin claim 12, wherein said substrate is provided with rake surfaces andflank surfaces, wherein said primary layer on said rake surfaces isprovided with tensile residual stresses which are not greater thantensile residual stresses in said primary layer on said flank surfaces.17. A coated hard alloy cutting tool, comprising:a substrate comprisingWC and Co, said substrate has a Co enriched surface layer of 50 μm fromthe external surface of said substrate and a core, the Co concentrationof a region that is 5-10 μm from the external surface is from 15-25 wt%, the Co content of said surface layer is more than that of said core,the content of carbides of Ti, Ta and Nb in the surface layer is lowerthan the content of said core; and at least one coating deposited onsaid surface of said substrate, said coating, sequentially consistsof:(a) a primary layer comprising a TiCN layer; (e) a secondintermediate layer comprising at least one layer selected from the groupconsisting of a TiC layer, a TiCO layer and a TiCNO layer; (b) asecondary layer comprising an Al₂ O₃ layer; and (c) a surface layercomprising at least one layer selected from the group consisting of aTiCN layer and a TiN layer; wherein said substrate is formed with rakesurfaces and flank surfaces; wherein said primary layer on said rakesurfaces is treated to produce compressive residual stresses of not morethan 20 Kg/mm² ; wherein said primary layer on said flank surfaces isproduced so that tensile residual stresses therein are not more than 20Kg/mm² ; and wherein said layers (a)-(c) and (e) are deposited bychemical vapor deposition.
 18. A coated hard alloy cutting tool,comprising:a substrate comprising WC and Co, said substrate has a Coenriched surface layer of 50 μm from the external surface of saidsubstrate and a core, the Co concentration of a region that is 5-10 μmfrom the external surface is from 15-25 wt %, the Co content of saidsurface layer is more than that of said core, the content of carbides ofTi, Ta and Nb in the surface layer is lower than the content of saidcore; and at least one coating deposited on said surface of saidsubstrate, said coating, sequentially consists of:(d) a firstintermediate layer comprising a TiN layer; (a) a primary layercomprising a TiCN layer; (e) a second intermediate layer comprising atleast one layer selected from the group consisting of a TiC layer, aTiCO layer and a TiCNO layer; (b) a secondary layer comprising an Al₂ O₃layer; and (c) a surface layer comprising at least one layer selectedfrom the group consisting of a TiCN layer and a TiN layer; wherein saidprimary layer is produced so that tensile residual stresses therein arenot more than 20 Kg/mm² ; and wherein said layers (a)-(e) are depositedby chemical vapor deposition.
 19. A coated hard alloy cutting tool asclaimed in claim 18, wherein a surface region within a distance of 100μm to 400 μm from said external surface is substantially free of freecarbon particles, while free carbon particles are present in a region ofsaid core located beyond 100-400 μm from said external surface of saidsubstrate.
 20. A coated hard alloy cutting tool as claimed in claim 18,wherein the thickness of said first intermediate layer is not more than1 μm.
 21. A coated hard alloy cutting tool as claimed in claim 18,wherein the thickness of said second intermediate layer of a TiCO layeror a TiCNO layer is not more than 1 μm.
 22. A coated hard alloy cuttingtool as claimed in claim 18, wherein the Co content in a region of saidcore located beyond about 100 μm depth from said external surface ofsaid substrate is within the range of from 4-8% by weight.
 23. A coatedhard alloy cutting tool as claimed in claim 18, wherein said substrateis provided with rake surfaces and flank surfaces, and wherein saidprimary layer on said rake surfaces is provided with tensile residualstresses which are not greater than tensile residual stresses in saidprimary layer on said flank surfaces.
 24. A coated hard alloy cuttingtool, comprising:a substrate comprising WC and Co, said substrate has aCo enriched surface layer of 50 μm from the external surface of saidsubstrate and a core, the Co concentration of a region that is 5-10 μmfrom the external surface is from 15-25 wt %, the Co content of saidsurface layer is more than that of said core, the content of carbides ofTi, Ta and Nb in the surface layer is lower than the content of saidcore; and at least one coating deposited on said surface of saidsubstrate, said coating, sequentially consists of:(d) a firstintermediate layer comprising a TiN layer; (a) a primary layercomprising a TiCN layer; (e) a second intermediate layer comprising atleast one layer selected from the group consisting of a TiC layer, aTiCO layer and a TiCNO layer; (b) a secondary layer comprising an Al₂ O₃layer; and (c) a surface layer comprising at least one layer selectedfrom the group consisting of a TiCN layer and a TiN layer; wherein saidsubstrate is formed with rake surfaces and flank surfaces; wherein saidprimary layer on said rake surfaces is treated to produce compressiveresidual stresses of not more than 20 Kg/mm² ; wherein said primarylayer on said flank surfaces is produced so that tensile residualstresses therein are not more than 20 Kg/mm² ; and wherein said layers(a)-(e) are deposited by chemical vapor deposition.
 25. A coated hardalloy cutting tool as claimed in claim 1, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 26. A coated hard alloycutting tool as claimed in claim 5, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 27. A coated hard alloycutting tool as claimed in claim 6, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 28. A coated hard alloycutting tool as claimed in claim 11, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 29. A coated hard alloycutting tool as claimed in claim 12, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 30. A coated hard alloycutting tool as claimed in claim 17, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 31. A coated hard alloycutting tool as claimed in claim 18, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 32. A coated hard alloycutting tool as claimed in claim 24, wherein said primary layer isdeposited at a temperature of 840°-900° C.
 33. A coated hard alloycutting tool as claimed in claim 1, wherein said primary layer isdeposited by reacting a mixture comprising titanium tetrachloride andacetonitrile.
 34. A coated hard alloy cutting tool as claimed in claim5, wherein said primary layer as deposited by reacting a mixturecomprising titanium tetrachloride and acetonitrile.
 35. A coated hardalloy cutting tool as claimed in claim 6, wherein said primary layer isdeposited by reacting a mixture comprising titanium tetrachloride andacetonitrile.
 36. A coated hard alloy cutting tool as claimed in claim11, wherein said primary layer is deposited by reacting a mixturecomprising titanium tetrachloride and acetonitrile.
 37. A coated hardalloy cutting tool as claimed in claim 12, wherein said primary layer isdeposited by reacting a mixture comprising titanium tetrachloride andacetonitrile.
 38. A coated hard alloy cutting tool as claimed in claim17, wherein said primary layer is deposited by reacting a mixturecomprising titanium tetrachloride and acetonitrile.
 39. A coated hardalloy cutting tool as claimed in claim 18, wherein said primary layer isdeposited by reacting a mixture comprising titanium tetrachloride andacetonitrile.
 40. A coated hard alloy cutting tool as claimed in claim24, wherein said primary layer is deposited by reacting a mixturecomprising titanium tetrachloride and acetonitrile.